Cisco ICND1 Flashcard: OSI Layer Model and PDU's

Pick up any CCNA Exam Preparation Guide. There are plenty enough books or chapters and webpages dedicated to the topic of OSI Layer Model of Internetworking. I'm just providing a short summary of notes to remember. It's guaranteed that there will be questions of the 7 Layer OSI model, the Cisco three layer, model, and even the DoD's model and how all the layers interrelate.

Physical Topology - Defines how the computer and networking devices are connected (physical)
Logical Topology - Describes the path the signals travel from one path to another (routing, tunnels: layer 2 for ipsec, layer 4 for ssl)

ESSENTIAL FACTS
* Each layer of the OSI model is only interested in communicating with its peer later at the destination.
* Each layer provides services to the layer above it.
* Encapsulation: as application data parses down the protocol stack to transmit across network media, each layer adds a header or trailer (containing addressing information).
* OSI model uses structure, forms a good reference model, permits change at one layer without affecting the other layers

Cisco Hierarchical Three Layer Model
Access - provides work group access for end users, desktop layer
Distribution - routing protocols and security, includes LAN based routers and layer 3 switches, enables routing between VLANs
Core - high speed and redundancy, provides high speed data transfer between sites

TCP/IP Stack - 4 Layers
Application - Application, Presentation, Session
Transport - Transport
Internet - Network
Network Access - Data Link, Physical

CCNA Illustrated OSI Layer Graphics

OSI Model - 7 Layers
* Including some quick notes on the pdu, addressing used
Application - data
Presentation - data
Session - data
Transport - segment, port number
Network - packet, ip address
Data-Link - frame, mac address
Physical - bits

Application Issues: Application, Presentation, Session
Data Transport, Implementation details: Transport, Network, Data-Link, Physical
Therefore, the de-encapsulation first occurs at the Transport leve.
Application Layer
* Authentication
* Examples: email, file transfer, terminal emulation

Presentation Layer
* Data representation, such as the format or structure of data (say jpeg, wav etc)
* Negotiates data transfer, syntax
* Provides encryption
* http, ssh operates at this layer
* It's safe to say, it makes sure that information sent at the application layer of one system is readable by the application layer of the other.

Session Layer
* Client - server connection
* Inter host communication
* Establish, manage, terminate sessions between applications. Such as the Shopping Cart?
* Web server, data exchange
* Examples: NFS, SQL, Netbios

Transport Layer
* Establishes end to end connections between hosts
* TCP and UDP ports
* The source port in the UDP Header and TCP Header is a 16 bit calling port.
* Reliability and flow control (windowing)
* Establish, maintain, terminates virtual circuits

Network Layer
* Primarily deals with data delivery
* Routes data packets
* Selects the best path to deliver data
* Provides local addressing and path selection
* Manages connectivity
* IP addresses to route packets

The Ethernet source and destination address is a 6-byte hex
Data Link Layer
* MAC address
* Error detection
* Does Encapsulation Frame Relay or PPP ring a bell?
* The most diverse

Physical Layer
* Examples: Copper, wireless, satellite, fiber
* 1, 0 bits transmitted by electrical pulse, electrons, light
* Think Cabling

CCNA Illustrated OSI Layer Model Graphics Blog